Some position detection systems related to touch screens sense the interruption of radiation (e.g., light) by an interposing opaque object (e.g., a finger, stylus, etc.). Such systems generally utilize radiation transmitters such as LEDs or IR emitters which are typically mounted in opposing corners of a same side of the touch screen. Each LED or IR emitter light source transmits a 90° fan-shaped pattern of light across the field of the touch screen, parallel to the viewing field surface.
A retroreflective sheeting material may be positioned around the perimeter of the active field of the touch screen, as disclosed in U.S. Pat. No. 4,507,557. The retroreflective sheeting material is generally arranged to reflect light received from the LED light sources back toward the originating source. Light incident on the front surface of the sheeting impinges on retroreflective elements, and is reflected back out through the front surface in a direction nominally 180 degrees to the direction of incidence. Digital cameras are located in the same opposing corners where the LED light sources are mounted to detect the retroreflected light that passes across the field of the touch screen and sense the existence of any interruption in this radiation by an opaque object.
One problem with the use of certain conventional retroreflective sheeting materials in touch screen applications and/or position detection systems is that dirt and/or moisture may penetrate the structure and adversely affect retro reflectivity of the retroreflective sheeting material. Another problem with conventional retroreflective sheeting material used in touch screen applications and/or position detection systems is difficulty in obtaining a uniform background throughout the area of interest (e.g., the detection area), against which the opaque object can be contrasted. Many conventional retroreflective sheeting material designs provide a non-uniform background and have portions, especially at or near the corner regions where the detected signal is very low. This makes it difficult to detect movement of the opaque object in such areas.
In operation, the position of the interposing object is typically determined by triangulation. When an interposing object such as a finger tip interrupts the pattern of light beams radiated from the LED light sources or IR emitters, a discrete shadow is created along a horizontal axis in the pattern of retroreflected light received by the two digital cameras. The digital cameras each generate a signal in which the discrete shadow registers as a dip in light intensity along a point of the horizontal axis of the camera field of view. A digital control circuit receives these digital camera signals and converts the horizontal position of the shadow into angles θ1, θ2 whose vertex originates with the digital cameras. Because the digital cameras are separated a known distance D at opposite ends of a same side of the touch screen, the y coordinate of the interposing object can be computed by the digital control circuit using the formula y=D/(1/tan θ1+1/tan θ2), and the x coordinate may be computed as x=y(1/tan θ1).